1. Field of the Invention
This invention relates to an ultrasonic doppler diagnostic apparatus, more particularly to an ultrasonic Doppler diagnostic apparatus capable of computing the velocity of movement of a moving reflective member within a subject under observation and accurately displaying such movement.
2. Description of the Prior Art
The ultrasonic pulse-Doppler method has been practically applied in the measurement of the velocity of movement of a body organ such as the heart, of blood flowing in the blood vessels, of other body fluids, or of the cardiac muscles. The velocity of movement is detected from the frequency shift of an echo reflected from the moving member within the subject.
The applicant previously filed an application for patent on a Doppler apparatus of this type constituted as a velocity computing apparatus based on autocorrelation, and this application was allowed as U.S. Pat. No. 4,573,477.
In the patented apparatus, the Doppler signal received from within the subject is converted to complex signals, autocorrelation of the complex signals is determined, and the frequency shift is computed from the autocorrelation value. The frequency shift obtained in this manner corresponds to the velocity of the moving reflective member so that it is possible to determine the velocity of the moving reflective member with high accuracy from the magnitude of the frequency shift.
The apparatus further displays the velocity of the moving reflective member on a CRT (cathode ray tube) in either B mode or M mode, in this way providing useful information for use in diagnosis through visual images.
When an ultrasonic Doppler apparatus employs only a single beam, however, accurate measurement of the velocity of moving a reflective member can be realized only for movement radial to the direction of the ultrasonic beam.
Because of this, it has been the practice to direct a plurality of beams originating from somewhat separated locations into the subject and then to obtain velocity information that includes the direction of movement of the moving reflective member by synthesizing the so-obtained plurality of velocity signals.
However, this method is disadvantageous not only in that it requires an apparatus of complex structure but also in that it cannot be applied when the moving reflective member is such that the transmission of the beams into the subject can be carried out only from limited locations or at a narrow range of angles. This problem arises particularly, for example, when the moving reflective member under observation is a cardiac muscle or the like.